Growth, spectroscopy and utilisation of novel low dimensional nanostructures: carbon nanotubes and quantum dots

Abstract

The work presented in this thesis deals with two important low dimensional nanostructures: carbon nanotubes (CNTs) and quantum dots (QDs). In the part of the work related to CNTs a novel method for growing CNTs without the need of metal catalyst is presented. The as produced CNTs were grown by means of chemical vapour deposition on Si-Ge islands and on Ge dots grown with the Stransky — Krastanow method on top of silicon substrates. Through rigorous characterisation products of the method were identified as single wall carbon nanotubes (SWCNTs) with diameters of 1.6 and 2.1 nm. Acquired Raman spectra showed very low intensity or none D — band while the G’ band was of high intensity indicating that the as produced CNTs may be of high quality. A by-product of this method is amorphous fibres which can be easily eliminated when exposed to HF vapour. As this method does not employ metal particles it is fully compatible with the front end silicon processing and therefore opens up the prospect of merging carbon nanotubes with silicon technology.
Furthermore CNTs were utilised as probes for atomic force microscopy (AFM). For the fabrication of the CNT probes two methods were applied successfully: the surface growth method and the pick up method. The latter was found to be substantially more efficient than the former and although not proper for mass production it is ideal for laboratory use as it can potentially generate thousands of CNT probes. The as fabricated CNT probes, had diameters in the range of 4 to 7 nm. Using CNT probes the surface of a mesoporous material with pore diameter of 7 to 12 nm and repeated distance of 15 to 18 nm was imaged, proving the high resolution that can be achieved with such probes and that AFM can be applied successfully to mesoporous materials. The latter has the potential to considerably expand the knowledge and the control of such materials to the nanoscale.
In the part of the work related to QDs a time resolved two colour pump photoluminescence (PL) technique was applied, with the aim to probe the coherent properties of the excitonic ground state of a single Stransky-Krastanow InGaAs QD. The method comprises of two pulses of different energy; a delayed blue pulse that pumps the GaAs barrier and an infrared (IR) pulse that pumps the excitonic ground state of the QD. The PL of the 1st excitonic excited state of the QD is used in order to probe the occupancy of the ground state. The detection is carried out at zero laser background and thus having a considerably higher signal to noise ratio than other pump and probe methods. A PL intensity variation and a red shift in the energy of the I excitonic excited state were observed, with both effects being dependant upon the intensity of the JR pulse but independent of the time delay and its energy. Further investigations showed that the IR excitation causes all PL and absorption lines of the QD to red shift, induces broadening of the absorption lines and increases the background absorption. Comparison with temperature dependent PL measurements showed that although heating might contribute to the above effects it cannot be the sole reason for their occurrence. Because of the above effects the time resolved two colour pump method cannot be applied as such for probing the coherence of QD ground excitonic state and needs to be modified further.

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